I've had the standard gastroporesis test at the hospital radiology department. It took most of the morning and was very VERY expensive! I was diagnosed with no gastroporesis.
I discovered a self test I can do myself to test my stomach emptying times.
I've have supplemented high dose pure niacin (not time release) for years and noticed variations in the timing of the unmistakable skin flush sensation depending on what or how much I eat.
i.e. just water on an empty stomach takes me 12-15 minutes for the niacin to hit the intestine and absorb causing the flush. With the standard test sample of 1 egg, 1 toast, milk, takes the niacin 20-30 minutes. Big dinner meals can take an hour or more. Some meals I eat slow the emptying so much that the niacin never creates the flushing because so little niacin hits the intestine quickly just like the time release versions of niacin that are designed so as not to flush.
Knowing what your emptying times can help with timing your boluses and learning which foods might be difficult to digest.
I know there are many who are leery of niacin supplements but I doubt it's as bad as the contrast radiation they use for the alopathic test.
I use a 1000mg pure niacin capsule. After years of niacin the flushing sensation has diminished but it's still unmistakable just not so uncomfortable.